Present work centers on improving blood compatibility of gas permeable membranes. We have found that pure gum silicone rubber is significantly superior to standard medical grade silicone rubber in terms of blood compatibility, and is especially useful in reducing platelet injury. Coating a conventional silicone rubber membrane with other polymers, such as fluorosilicones, or adding other types of fillers, is being investigated. Sheep hearts have been preserved at 13 degrees C by perfusing hearts with various fresh plasma fractions. We find significant enhancement of heart viability when at least some red blood cells are added to the perfusate.